Add support for a `verbose` parameter

[xavierfoucrier]

It would be cool to have a verbose parameter to prevent the plugin from "polluting" the console output with HandlebarsPlugin: + partial or HandlebarsPlugin: + helper or HandlebarsPlugin: + created output.

Note that this is an issue when you want to profile the webpack build using Webpack Bundle Analyzer CLI:
webpack --config webpack.dev.js --profile --json > stats.json will output HandlebarsPlugin log at the beginning of the stats.json file.